1. Field of the Invention
This invention relates to rotary well drilling, particularly to a shock absorbing apparatus placed in the drill string.
2. Description of the Prior Art
Shock absorbing apparatus is used to reduce the vibrations generated during rotary well drilling. One type of shock absorber is disclosed in U.S. Pat. No. 3,382,936, issued to the assignee of Edward M. Galle. It uses gas as the shock absorbing medium and a liquid separated from the gas by a flexible wall separator or compensator. In U.S. Pat. No. 3,746,329, Galle discloses a solid piston type separator that replaces the flexible wall separator and yet equalizes during drilling the pressure of the gas and liquid. This device is used for deep oil well drilling, during which the drilling mud transmits the borehole hydrostatic pressure to the liquid and gas in the shock absorber.
Another flexible separator or compensator is used to separate the drilling mud and lubricant inside Galle's apparatus. If the separator ruptures or fails, the entire tool may fail since the abrasives normally present in the mud are extremely destructive.
The gas chamber in Galle's apparatus is long since hydrostatic pressure increases as the tool is lowered in a liquid filled borehole and causes compression of the gas. The large weights applied to the bit cause additional compression. It would be advantageous to shorten the length of a shock absorber used in blast hole drilling.
Normally in blast hole drilling the drilling fluid is air, and thus there is atmospheric but no hydrostatic pressure in the well bore. Because of small hole diameters, it is frequently advantageous to place the shock absorber above the hole at the drill. Consequently, the tool must be relatively short in comparison with the length of Galle's apparatus.
Also, the Galle apparatus if used in blast holes where air is the drilling fluid would experience large pressure differentials across the seals located between its body and reciprocal mandrel. Since the frictional pressure of a sliding seal is proportional with pressure differential across the seal, excessive heat might result, with consequent seal deterioration.
The splines used to transmit rotary motion from the tubular body to the reciprocable mandrel in the Galle apparatus are difficult to machine to that degree of accuracy required to prevent localized wearing and galling. Seals may be damaged in the presence of the metallic particles produced by such wearing or galling.
Since a shock absorber for blast hole operations would normally be operating above hole, it is desirable to have a provision for re-greasing the bearing areas without affecting the initial charge pressure in the gas cavity or disassembling the tool.